While it’s getting the most attention, the $4.1 million IRS conference in California in 2010 that has Congress up in arms isn’t even the costliest event the tax agency hosted over the last few years.

When calculated on a per-person basis, the Anaheim conference’s $1,600-per-attendee price tag is only the 46th most-expensive one the agency has held in the past three years, and is dwarfed by the $5,600 paid for each attendee at a 2012 training session for new employees.

Two other costly training sessions in 2012 came to nearly $5,000 and $3,600 a person, according to a Washington Times analysis of data from an inspector general’s report this week highlighting high-dollar IRS conferences over the past three years.

On Wednesday, the IRS put two officials on administrative leave for their involvement in the 2010 California conference.

The scrutiny over the conferences comes as the IRS is already on the defensive over its targeting of conservative groups for invasive questions about their membership and activities and for “slow-walking” their applications for tax-exempt status.

An inspector general’s report released this week highlighted the $4.1 million California conference, but also pointed to more than 200 other conferences the agency held over the last two years that had 50 or more employees in attendance.

Sen. Tom Coburn, Oklahoma Republican and Congress’ top waste-watcher, sent a letter to Treasury Secretary Jack Lew this week asking why his department concealed those conferences from him in 2012, when he specifically asked for a list of all department events with 50 or more people.

The Treasury Department, which oversees the IRS, sent Mr. Coburn a list of just five conferences, with a total cost of $500,000, none of which involved the IRS. This week’s audit showed more than 150 conferences in 2010 alone, with a total cost of $37.6 million.

Mr. Coburn said he has referred the matter to investigators to see why Treasury concealed the IRS conferences from him.

The House oversight committee will hold a hearing Thursday on the conferences, where the Anaheim event is likely to dominate.

Inspector General J. Russell George, who is slated to testify at the hearing, identified a number of potential problems with the California conference, including spending $15,669 on commemorative tote bags, $19,210 on lanyards, travel mugs and picture frames with the IRS division’s logo, and $27,000 on promotional giveaways such as engraved pens, can coolers and “Post-it” notes.

He said IRS employees accepted room upgrades in exchange for paying the full per diem nightly rate, when they could have instead taken regular rooms and tried to negotiate down the price.

The IRS also violated its own tax rules by letting some local California employees stay at the posh Anaheim hotels without requiring them to pay taxes on the per diem expenses.